The present invention relates to the field of devices and methods for finding exact translations and rotations within computer controlled machines such as machine tools, opto-mechanical measuring devices, coordinates measuring machines (CMM), robots, and as position encoders. More specifically the invention relates to the rigid body translation and rotation calibration at a plurality of machine part positions, or simultaneous translation and rotation reading within such devices.
Referring to XYZ coordinate axes, by the translation of a part is in the following meant the X,Y,Z coordinate of a specific location on that part. By the rotation of the same part is meant the Rx,Ry,Rz rotation angles of that part, where Rx, Ry, and Rz refer to rotation angles around the given X, Y, and Z axes, respectively. By the position of a part is in the wider sense meant either the combined translation and rotation, the translation, or the rotation of a part. Typically the above mentioned machines use translation reading devices, so called encoders, to read the exact translation of different parts of the machine. If, for example, such a machine is built with three translation degrees of freedom, XYZ, and one rotation angle degree of freedom, θ, linear encoders are placed at each of three respective X, Y and Z-carriers, and an angular encoder is placed at a rotation axis of a θ carrier of the machine. However, these encoders are usually located at a distance from the work area (or work space) of the machine; otherwise the encoders would come in conflict with the work area operations of the machine. As a consequence, in order to determine the translation of a specific machine part or tool in said workspace, translations and rotations of several machine parts need to be determined from measurements made by the respective encoders. By using geometrical information and performing geometrical calculations based on said measurements, the translation and rotation of said specific machine part, or typically the translation and rotation of a tool located in the work space of the machine, is derived. However, mechanical irregularities, clearance, and/or mechanical play, affect machine part movements. Thus, translation and rotation offsets between the encoder reading positions and the work area operation positions, introduce hard-to-measure offsets associated with each respective degree of freedom, whose offsets are not accounted for in said geometrical calculations, and which in its turn leads to a certain degree of uncertainty and error of the determined machine part positions.
In order to measure and calibrate the 3D (three dimensional) positioning of e.g. machine tools, opto-mechanical devices, and encoders, so called touch probes are typically used. A touch probe can be mounted into the machine tool tool-holder and, for measurement purposes, be moved to touch the calibrated position of gauges like steel balls or bars. This is a time consuming point by point process and requires that expensive dimension calibrated gauges are mounted and used.
Typically an encoder measures the 1D (one dimensional) translations along a bar or, to read a rotation angle, the 1D rotation on a periphery of a rotating shaft. It may be complicated and expensive to expand the same processes to simultaneously read both translations and rotations for some, or all of the 6 (3 translations+3 rotations) possible mechanical degrees of freedom of a rigid body. Present day encoders may, due to accuracy limitations, not be very suitable for reading the difference between e.g. the translations along two bars and possibly extrapolate those difference translations into values for translations for locations reaching far outside the bars.
In the literature, such as Christopher J. Dainty ed. in Laser Speckle and Related Phenomena, Springer Verlag, Berlin 1974, a range of so called speckle photography and speckle interferometry techniques are described. The main focus of those techniques is on the measurement of object-internal deformations and surface topography. The speckle photography techniques are not able to measure both local translation and rotation angle offsets at a plurality of part positions in the 3D space. Correspondingly, and in addition, interferometric techniques are vibration sensitive, and in many cases not well suited for industrial applications.
Later on, e.g. Ichirou Yamaguchi et. al. in Applied Optics, Vol. 28, No. 20, Oct. 15, 1989 and Vijay Shilpiekandula in his Master thesis, Massachusetts Institute of Technology, February 2004, describe how a defocused or focused camera can be used to make a rotation angle reading encoder by recording the speckle displacement by use of eq. a camera. This technique also lacks the ability to measure both local translation and rotation offsets at a plurality part positions in the 3D space.
The European patent EP1924400, describes an apparatus and method for finding part position relations of parts of mechanical and opto-mechanical machining and quality control systems, and for recognizing these parts. This technique describes, amongst others, correlation techniques to find image displacement of focused surface structure. But this technique lacks the ability to measure both translation and rotation offsets at a plurality of part positions in the 3D space.
Thus, known mechanical and optical devices and methods, for finding translation and rotations within computer controlled machines, lack sufficient measurement ability or are too sensitive or error-prone. Further they typically require time-consuming and/or expensive calibration.